To stop an elevator smoothly and level with a sill, an elevator system must know when to initiate a stop, when to go into a leveling mode of operation, and when to begin opening the landing doors. Most elevators begin opening their doors two to three inches before the elevator car is actually level with the sill to speed-up passenger transfer. This area is known as a door zone. The elevator doors must not be opened when the elevator car is not within the door zone. It is therefore necessary to know the exact location of the elevator car at all times. As a consequence, elevator position devices are used to monitor elevator car position.
One existing elevator position device includes steel bars, vanes or magnets attached to a floating steel tape, running the length of the hoistway, and a hoistway position reader box mounted on the car which are used to monitor the car position. The steel bars, vanes or magnets are located on the steel tape with respect to their corresponding landing sills to mark the approximate distance from the door zone. The reader box contains sensors that sense the location of each steel bar, vane or magnet as the car travels up and down the hoistway such that the elevator system may determine if the elevator car is within the door zone corresponding to a particular landing. Two sensors are used to protect against a false door zone detection in the event that one sensor fails in an active state. The sensors are connected to an AND gate, the output of which is used to determine if the elevator car is within the door zone. For example, if both sensors are actuated (i.e., in the active state) the AND gate produces a logic "1" which represents the presence of a door zone and allows the elevator system to open the elevator doors.
Other techniques for determining if an elevator car is positioned within a door zone are sought, and it is to this end that the present invention is directed.